Agreed, although most of this will end up wrapped up into the token and system itself, I suspect. U2F is pretty much a "key" (some even visually looking like k

Agreed, although most of this will end up wrapped up into the token and system itself, I suspect. U2F is pretty much a "key" (some even visually looking like keys) that are used pretty much like a physical key - put the key into the keyhole (USB port), and press the flashing light. Done. That level of UX is what we all need to build towards!